Java review

Organized by Java: How to Program chapter.

1. About Java - history, compilation, etc.
2. Applications, first program, arithmetic, memory.
3. Applets, drawing, floating point.
4. Conditional statements - if, else, while ++, --
5. Conditional statements - for, do...while, switch, break, continue.
6. Methods
7. Arrays
8. Object-Oriented Programming
13. Graphical User Interface Components I

Typical Java Compilation/Execution Steps

1. Edit source (.java) program (eg, your IDE)
2. Compile source program to byte-codes (.class).
3. Load into RAM using the Java Class Loader.
   • Verify correctness of byte codes.
   • Allocate stack, heap (classes and statics).
   • Start JVM (Java Virtual Machine) at main (application).
4. JIT. JVM may translate a method from byte code to machine code the first time it's called (Just In Time compilation).
5. Execute the machine code.
6. The server version of the Java run-time system may recompile from byte code to machine instructions after analyzing the performance (HotSpot).

Implications of Java Byte Code

Java Byte Code (JBC) is interpreted/translated by the Java Virtual Machine (JVM).

• Why compile to Java Byte Code instead of machine code?
• Why use JIT compilation instead of an interpreter?
• Is Java slower, same, or faster than eg C++ because of Java Byte Code.
• How does JBC affect portability?
• How does JBC affect distribution as eg Applets or WebStart?
• Can other languages be compiled into Java Byte Code?
  See www.robert-tolksdorf.de/vmlanguages.html.

Design Patterns

Design patterns are "proven architectures for constructing flexible and maintainable object-oriented software". They:

• Promote design reuse.
• Help solve common problems.
• Help avoid common mistakes.
• Establish a common vocabulary among developers.

A First Program with Console Output

Similar to JHTP p 34 without comments

public class Welcome1 {
    public static void main(String[] args) {
        System.out.println("Welcome Earthlings");
    }
}

• What file is this in?
• What is the alternative way to declare the array parameter?

Program using Dialog Box

// Similar to JHTP p 43 without comments.
import javax.swing.JOptionPane;

public class Welcome2 {
    public static void main(String[] args) {
        JOptionPane.showMessageDialog(null, "Welcome Earthlings");
        
        System.exit(0);
    }
}

I don't expect you to know the answers to these - this time!

• Give two alternatives to the import.
• Why is System.exit(0) necessary here, but not in previous example?
• What does the zero mean in System.exit(0)?
• What would often replace the null in this example?
• Is JOptionPane a class or an object?
• Methods that are are qualified with a class are called which? static or instance?

Converting Strings to Integers

// Similar to JHTP p 47 without comments.
import javax.swing.JOptionPane;
public class Add2 {
    public static void main(String[] args) {
        String s1, s2;
        int    n1, n2;
        s1 = JOptionPane.showInputDialog(null, "Enter 1st number");
        n1 = Integer.parseInt(s1);
        s2 = JOptionPane.showInputDialog(null, "Enter 2nd number");
        n2 = Integer.parseInt(s2);
        
        JOptionPane.showMessageDialog(null, "Sum is " + (n1 + n2));
        
        System.exit(0);
    }
}

• What happens if the user enters something that's not an int?
• Are the parentheses around (n1 + n2) necessary?

Arithmetic Operators

Basic arithmetic operators: +, -, *, /, %

Precedence: *, /, and % are done before + or -.

If either operand of + is a string, the other operand is converted to string and the two strings are concatenated.

Arithmetic Comparison Operators

Comparison operators: <, <=, ==, !=, >=, >

Precedence: All are lower precedence than the arithmetic operators.
<, <=, >=, > are higher than ==, !=, but there is no apparent reason for this except compatibility with C and C++.

The result of any comparison is boolean true or false.

Logical Operators

• boolean true or false is the result of all logical operators.
• "Short circuit" operators (&&, ||): They do not need to evaluate both operands when result is known. Best choice most of the time.
• And - &&, & - true only if both operands are true.
• Or - ||, | - true if either of the operands is true.
• Not - ! - true if operand is false, false if operand is true.
• Xor (exclusive or) - ^ - true if the operands are different, false if they are the same.
• Precedence: comparisons > & > ^ > | > && > ||
  In practice, remember they are lower than comparison, but always use parentheses when mixing logical operators.

End